As illustrated in FIG. 13A, a lead terminal 202 of an electronic component 201 is inserted into a through hole 102 of a printed substrate 101 and the printed substrate 101 passes through a surface of a molten solder 301 to solder the electronic component 201 soldered to the printed substrate 101. Such soldering is called flow soldering. As the molten solder 301 rises inside the through hole 102 and the molten solder 301 is cooled, as illustrated in FIG. 13B, a solder 302 is formed inside the through hole 102, and the lead terminal 202 of the electronic component 201 is soldered to the through hole 102. A state in which the molten solder 301 rises inside the through hole 102 is referred to as molten soldering.
In the recent years, from the viewpoint of environmental problems, lead-free solder is used in place of lead-containing solder. Since a melting point of the lead-free solder is higher than the melting point of the lead-containing solder, soldering using lead-free solder is performed at a temperature higher than a soldering temperature using the lead-containing solder. In a case where a large-size component is soldered to the substrate, since the large-size component easily absorbs heat, the heat of the molten solder is absorbed by the large-size component. A wiring pattern (solid pattern) is formed on the surface of the substrate by etching copper foil provided on the surface of the substrate. In addition, in a case where a substrate is formed by stacking insulating layers on which wiring patterns are formed, the wiring pattern is provided inside the substrate. Since a thermal conductivity of the copper foil is high and the wiring pattern is arranged spreading in a planar direction of the substrate, the heat of the substrate is radiated through the wiring pattern. In a case where the wiring pattern is arranged in a vicinity of the through hole, the heat of the molten solder is absorbed by the wiring pattern. When the temperature of the molten solder decreases, a fluidity of the molten solder decreases and the molten soldering is inhibited. When the molten soldering is inhibited, there is a possibility that the surface of the substrate and the lead terminal of the electronic component are soldered in an insufficient state.
There are attempts to improve the molten soldering by changing the process such as raising the heating temperature of the solder or prolonging a heating time of the solder. Since it is possible to change the process by changing a setting of a heating device, implementation is easy. However, since the temperature of the other components around the through hole and the substrate further increases, other parts and the substrate may be destroyed.
There are attempts to improve the molten soldering by changing the design of the substrate. As the changing the design of the substrate, there are,
(1) Enlargement of the diameter of the hole of the through hole,
(2) Deleting of the connection portion between the wiring pattern and the through hole, and
(3) Deleting of the wiring pattern itself,
for example.
However, even when the diameter of the hole of the through hole is enlarged or the connecting portion between the wiring pattern and the through hole is deleted, the molten soldering hardly improves. In addition, when connecting portion between the wiring pattern and the through hole is deleted or wiring pattern itself is deleted, the electric characteristics of the substrate may be deteriorated.
There are attempts to improve the molten soldering by soldering the components to the substrate using the solder having a low melting point. For example, the molten soldering is improved by soldering the component to the substrate, using solder having a melting point of 140° C. from a solder having a melting point of 220° C. However, the solder having the low melting point tends to be inferior in connection reliability against vibrations, impacts, and the like.
The followings are reference documents.
[Document 1] Japanese Laid-open Patent Publication No. 2004-273990,
[Document 2] Japanese Laid-open Patent Publication No. 2007-42995,
[Document 3] Japanese Unexamined Utility Model Registration Application Publication No. 2-13770 and
[Document 4] Japanese Unexamined Utility Model Registration Application Publication No. 5-72176.